Climate Outlook

NORTH AMERICA July - December 1998

The IRI Experimental Forecast Division has prepared this Climate Outlook for North America for July - December 1998. Of relevance in the preparation of this Outlook have been the recent development of cooler than average conditions in the central equatorial Pacific Ocean, together with an area of substantial positive anomalies off the west coast of South America. The tropical Atlantic and Indian oceans are currently warmer than average, but the Indian Ocean and South Atlantic Ocean are expected to cool.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST covering the forecast period. Particularly heavy weighting has been given to predictions from the coupled model operated by the NOAA National Centers for Environmental Prediction, Climate Modeling Branch. This model suggests the continued development of weak to moderate La Niņa conditions, with the persistence of strong positive sea-surface temperature anomalies in the region of Indonesia.

B) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted sea surface temperature (SST) patterns.

C) Statistical analyses of the observed climate over North America.

D) Mesoamerica Climate Outlook Forum and Caribbean Regional Climate Outlook Forum consensus guidances.

E) Other sources of information include the Caribbean Meteorological Institute, CPTEC, ECMWF, NCEP, Universidad Nacional Autonoma de Mexico, University of the West Indies.

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in North America. Thus, this product may differ from the official forecasts issued in those areas. This Outlook has been prepared in the time available, using all information that was reasonably accessible. Inclusion of other climate information and guidance requires further arrangements. The IRI is engaged in establishing such collaborative arrangements with the goal of improving its capability to provide the best and most complete global climate guidance. The Climate Outlook for July - December 1998 is dependent on the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is considerable spread in coupled model predictions concerning the evolution of SSTs. This spread in predictions is a primary source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will continue to decline at a modest rate, resulting in moderate La Niņa conditions by the end of the year. Also, it is known that Atlantic SSTs may play some role in modulating precipitation changes over parts of North America. Thus, the uncertainties in the Atlantic SST values during the forecast period may lead to additional uncertainty over some parts of the region.

It is stressed that the current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local variations should be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

This Outlook covers two seasons: July - September 1998 and October - December 1998. Maps are given showing expected tercile probabilities of precipitation. The maps indicate the probabilities that the seasonal precipitation will fall into the wettest third of the years (top number), the middle third of the years (middle number), or the driest third of the years (bottom number). An additional map is provided for the first season indicating probabilities for extreme precipitation anomalies. Extremes are defined as anomalies that fall within the top and bottom 15th percentile of the observed records. Maps of temperature show expected probabilities that the seasonal temperatures will fall into the warmest third of the years, the middle third of the years, or the coldest third of the years. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas marked "D" experience their dry season during the forecast period, typically receiving less than 15% of their annual precipitation total during the three months. Otherwise, for example, in the case of Arizona and Utah in July - September 1998 (Map A), there is a 25% probability that the precipitation will be in the wettest third of the years, a 30% chance it will be in the near- normal third of the years, and a 45% chance that the precipitation will be in the driest third of the years. The numbers for each region on the temperature maps indicate the probabilities of temperatures to fall in each of the three categories, above-, near-, and below-normal. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

July - September 1998:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.A) slightly enhanced probabilities for above-normal precipitation over the north-western part of Canada and eastern part of Alaska;

P.B) elevated probabilities for drier than normal conditions over Arizona, Utah, eastern parts of Nevada and California;

P.C) enhanced probabilities for normal precipitation over Ontario and western parts of Quebec provinces in Canada;

P.D) elevated probabilities for above-normal precipitation over the eastern tier of the United States;

P.E) enhanced probabilities for wet conditions over Mexico and central America, and there is also a slight enhancement of risk for extreme above-normal precipitation (within the wettest 15% of years in the observed record) over the northern half of Mexico. See map A2.

P.F) probabilities slightly favor near-normal to below-normal conditions over the Caribbean coast of Central America and over the Caribbean islands.

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.A) enhanced probabilities for warmer temperatures over Alaska and northern and western parts of Canada, especially over Quebec, Nova Scotia and Main;

T.B) elevated probabilities for above-normal temperatures over the southern tier of the United States, northern Mexico and the Caribbean;

T.C) highly enhanced probabilities for above-normal temperatures over Central America.

October - December 1998:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.i) elevated probabilities for drier than normal conditions over the western part of the United States;

P.ii) probabilities slightly favor below-normal conditions over most of Texas, Louisiana, parts of Oklahoma, Arkansas and Mississippi.

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.i) enhancement of probabilities for colder temperatures over western Canada and Alaska;

T.ii) probabilities slightly favor above-normal temperatures over the eastern half of the United States, Ontario and western part of Quebec.

Cutoffs and Max/Min extremes for JAS and OND.